Lead filled ceramic media for grinding



May 15, 1962 E. P. CADWELL LEAD FILLED CERAMIC MEDIA F OR GRINDING Filed Sept. 29, 1953 ATTO R N EY United States Patent 0.

3,034,735 LEAD FILLED CERAMIC MEDIA FOR GRINDING Edward P. Cadwell, Santiago, Chile, assignor to American Cyanamid Company, New York, N.Y., a corporation of Maine Filed Sept. 29, 1953, Ser. No. 383,055 Claims. (Cl. 241-184) This invention relates to the crushing and grinding of solids and more particularly to a grinding medium for tumbling mills. It also relates to a process of grinding solids in tumbling mills,

The crushing and grinding of various solids often requires the use of tumbling mills. Tumbling mills is a generic name sometimes used referring to ball, pebble, rod, tube, and compartment mills, ecause of the action of the grinding medium. These mills are usually made up of a s-teelor stone-lined cylindrical shell, containing a charge of steel rods or balls or stone pebbles. On rotation of the cylindrical steel shell crushing and grinding is eifected by the tumbling of the balls or pebbles on the material between them.

Although steel or iron balls or rods are commonly used in tumbling mills, their use is prohibited in certain grinding operations. For example, where contamination of a comminuted product by iron is not permissible, steel or iron balls cannot be used. Or where corrosive action such as oxidation is particularly acute iron or steel grinding media may not be employed.

To overcome these handicaps it has been suggested that fiint stones be used as a grinding media. However, the low specific gravity of flint stones has limited their grinding capacity exceedingly. Thus, flint stones are unsatisfactory for many grinding operations.

It has also been suggested to prepare grinding media, particularly balls, by coating a lead core with rubber. Such a grinding medium has the advantage of high specific gravity and high resistance to corrosion. Rubbercoated lead balls serve very well for grinding soft solids suchas vegetable. or plant material. However, many types of grinding operations, for example, the grinding of quartz, call for a hard, brittle surface on the grinding medium. Rubber-coated lead balls fail in this respect. Furthermore, many grinding operations are carried out in the presence of a liquid, usually water. If, however, the grinding is to be carried out in the presence of a liquid such as, for example, a hydrocarbon, then rubber-coated grinding media are unsatisfactory since the rubber may dissolve in such solvents. Also, Where lead is used as a core in a grinding medium it is desirable that the lead be readily recovered from the coating material. The remnants of a rubber coating on such grinding media are often hard to remove.

It is, therefore, the principal object of the present invention to develop a grinding medium not handicapped by the shortcomings mentioned above. It is a further object to develop grinding media suitable for grinding hard solids. It is a further object to develop a grinding media having a high specific gravity and high corrosion resistance under the conditions of operation.

The objects of the invention have been accomplished by developing a grinding medium wherein a core made of lead is surrounded with an outer wearing surface made of a hard ceramic. The grinding media and the process of using it will be more completely described with reference to the accompanying drawing in which:

FIG. 1 is a perspective view of a grinding ball of the present invention;

FIG. 2. is a vertical sectional view taken along line 22 of FIG. 1;

FIG. 3 is a vertical sectional view taken along line 3-3 of FIG. 1;

3,034,735 Patented May 15, 1962 ice FIG. 4 is a broken perspective view of a grinding rod shell; and I FIG. 5 shows a lead-filled grinding rod shell partly in section.

FIG. 1 shows a grinding ball covered with a hard ceramic 1. The holes 2 in the ceramic shell 1 allow lead to be poured into the interior of the ceramic shell. Usually two holes as illustrated in FIG. 1 are better than one since entrapped gases and air inside the shell can escape from one hole as lead is being poured in the other. In FIG. 2 the shell of the ceramic 1 is shown surrounding the lead core 3. The thickness of the ceramic 1 is immaterial since it may be adjusted at will. Since the specific gravity of the grinding ball decreases as the thickness of the ceramic increases, it is possible to adjust the specific gravity of the grinding ball to any desired value. Although the specific gravity can be adjusted to any value between that. of uncoated lead to that of the unfilled ceramic shell, it is usually desirable to achieve the specific gravity of iron or steel balls which runs around 7. The specific gravity of 7 is usually excellent in practice.

FIG. 3 shows a shell of ceramic 1, the holes 2 for filling and the lead core 3. As shown the lead core 3 fills the holes 2 to the surface of the ceramic 1. This can be achieved in filling the shell of ceramic 1 with molten lead to the capacity of the shell and then allowing the lead to cool. This will result in shrinkage of the lead. Additional lead can then be poured in the holes in order that the lead will completely occupy the holes 2 to the outer surface of the shellof ceramic 1. The filling of the holes 2 with lead 3 helps minimize any chipping that might tend to take place at or near the site of the holes 2. Any other suitable method of completely filling the ceramicshell with lead may be used such as using a lead alloy which does not shrink when the liquid metal cools and turns solid.

FIG. 4 shows a shell of ceramic in the form of a tube. Although this tube 4 can be made with the ends 5 closed and thenfilled with lead through the holes 6, it-is pref.- erable not to do so. Instead, the tube 4 is cast of a suitable ceramic, the tube is placed on one end 5 thus sealing that end, and lead is poured in the other end 5. In FIG. 5 the tube 4 is filled with an inner core of lead 7. The end 5 of the core of lead 7 need not be covered with ceramic since the end 5 is not part of the grinding surface of the tube 4. With open-ended rods the lead core adheres better if a lead alloy is used that does not shrink as it passes from the liquid phase to the solid phase on cooling.

By ceramic is meant any clay or siliceous product brought by the action of heat to a sufficient hardness to serve in a grinding media; the term also includes certain metal carbides such as silicon carbide, and certain metal oxides such as zirconium oxide. The surface of the ceramic may be porous or vitreous, glazed or unglazed. One of the outstanding advantages of the present invention is that the shell may be made of a ceramic especially selected for a specific grinding job. Thus, if quartz is to be ground a ceramic containing high silica may be used to minimize contamination of a product. The ceramic shells for the balls and tubes utilized by the present invention may be prepared by any of the means for preparing hollow objects well known in the art.

One of the surprising aspects of the grinding media of the present invention is the strength shown by the media. Since the lead core completely fills the ceramic shell the balls or tubes can withstand a surprising amount of rough handling. There is very little chipping of the ceramic in actual practice.

Another outstanding advantage of the present invention is that the lead cores of the grinding media may be readily recovered when the ceramic shell has become worn away. Any remaining ceramic covering may be readily removed from the lead core by heavily striking the worn grinding media with a hammer, or by passing the media through a jaw crusher or bowl mill. Since the lead is easily malleable, the ceramic readily breaks away under such treatment. Recovery of lead runs on the order of 90%. Thus the practice of this invention becomes peculiarly advantageous in those remote regions which are difiicult of access and where shipping costs are :high. Once a supply of lead has been accumulated at the grinding site only the ceramic shells need be shipped into the region; the lead may be easily remelted and used over and over. Appreciable savings in shipping costs can thus be realized.

The grinding media of the present invention are particularly useful in grinding operations where corrosion is excessive and where contamination due to the oxidation of iron balls is detrimental. The lead-filled ceramic grinding media of the present invention are exceedingly inert and innocuous in grinding mill processes. Excellent results can be obtained in processes of grinding solids in tumbling mills in which the lead-filled ceramic is used.

The new grinding media may be made in any desirable size since it is often advantageous to utilize balls or rods of varying diameters during a grinding operation. The inertness of the new grinding media of the present invention is of particular value in grinding wet material where the grinding media is submerged in water or other liquid. Tumbling mills can be operated in the same manner with the new grinding media as they are with any of the grinding media of the prior art.

I claim:

1. A corrision resistant grinding medium for use in tumbling mills consisting essentially of an outer wearing surface shell of a hard ceramic material, said shell having a plurality of adjacent openings therethrough, and a lead core entirely filling said shell and said openings; the volume of said core and of said shell being so proportioned as to obtained a predetermined average specific gravity.

2. A grinding medium according to claim 1 in which said grinding medium has an average specific gravity of approximately seven.

3. A corrosion resistant grinding ball for use in ball mills consisting essentially of an outer wearing surface shell of a hard ceramic material, said shell having a plurality of adjacent openings therethrough, and a lead core entirely filling said shell and said openings; the volume of said core and of said shell being so proportioned as to obtain a predetermined average specific gravity.

4. A corrosion resistant grinding rod for use in rod mills consisting essentially of an outer wearing surface shell of a hard ceramic material, said shell having a plurality of adjacent openings therethrough, and a lead core entirely filling said shell and said openings; the volume of said core of said shell being so proportioned as to obtain a predetermined average specific gravity.

5. A corrosion resistant grinding medium for use in tumbling mills consisting essentially of an outer wearing surface shell of a hard ceramic, said ceramic shell having two adjacent openings therethrough, and a lead core entirely filling said shell and said openings; the volume of said core and of said shell being so proportioend as to obtain a predetermined desired average specific gravity.

References Cited in the file of this patent UNITED STATES PATENTS 324,918 Caspersson Aug. 25, 1885 329,750 Low Nov. 3, 1885 504,508 Weswell Sept. 5, 1893 1,016,272 Johnson Feb. 6, 1912 1,247,545 Kittle Nov. 20, 1917 1,335,269 Ball Mar. 30, 1920 1,640,885 Curtis Aug. 30, 1927 2,204,582 Donahue June 18, 1940 2,489,307 Miller Nov. 29, 1949 2,518,758 Cook Aug. 15, 1950 2,653,769 Ha-ll Sept. 29, 1953 FOREIGN PATENTS 2,553 Great Britain Feb. 12, 1891 26,556 France Oct. 9, 1923 OTHER REFERENCES Chemical Engineering, September 1950, pages 152 and 154, article, Ball Mill Media. 

1. A CORROSION RESISTANT GRINDING MEDIUM FOR USE IN TUMBLING MILLS CONSISTING ESSENTIALLY OF AN OUTER WEARING SURFACE SHELL OF A HARD CERAMIC MATERIAL, SAID SHELL HAVING A PLURALITY OF ADJACENT OPENINGS THERETHROUGH, AND A LEAD CORE ENTIRELY FILLING SAID SHELL AND SAID OPENINGS; THE VOLUME OF SAID CORE AND OF SAID SHELL BEING SO PROPORTIONED AS TO OBTAINED A PREDETERMINED AVERAGE SPECIFIC GRAVITY. 